From mboxrd@z Thu Jan 1 00:00:00 1970 From: AL-KERNEL To: kernel-cve@kernelcve.org Subject: [CVE-2026-64383][IMPORTANT] smb: client: fix double-free in SMB2_flush() replay Date: Sat, 25 Jul 2026 15:44:34 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-AL-KERNEL-CVE: CVE-2026-64383 X-AL-KERNEL-Priority: IMPORTANT X-AL-KERNEL-Severity: IMPORTANT X-AL-KERNEL-Base-Severity: IMPORTANT X-AL-KERNEL-KPANIC: YES X-AL-KERNEL-ActionableScore: 8 X-AL-KERNEL-ActionableScore-Lower: 6 X-AL-KERNEL-Commit: 6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb List-Id: CVE: CVE-2026-64383 Priority: IMPORTANT AL-KERNEL base severity: IMPORTANT KPANIC flag: YES Patch: smb: client: fix double-free in SMB2_flush() replay Commit: 6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64383 Analysis date: Sat, 25 Jul 2026 15:44:34 -0400 ActionableScore: 8 ActionableScore lower bound: 6 Actionable bucket: Strong Important candidate / Actionable Moderate at minimum Manual review required: YES Summary: A double-free in the Linux SMB client SMB2_flush replay path can be triggered by a malicious or compromised SMB server through stale response buffer state across replay attempts, causing at least kernel DoS and requiring manual review for possible memory-corruption impact. ====================================================================== ABOUT THIS REPORT ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64383 is available here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64383 The original announcement does not normally provide a security severity estimate, CVSS assessment, or enough information to determine whether the reported kernel bug represents a practically relevant security issue. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. It combines an autonomous classifier with LLM-assisted technical analysis and a separate ActionableScore mechanism. The purpose of this report is to prioritize Linux kernel CVEs before manual review, identify cases that require prompt investigation, and support automatic closure of issues that are unlikely to have meaningful security impact. Published priority for this report: IMPORTANT Manual review required: YES A detailed explanation of the methodology and priority rules is included at the end of this message. ====================================================================== AL-KERNEL CLASSIFICATION RESULT ====================================================================== CVE-2026-64383 IMPORTANT CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-415;*CWE-416;*CWE-664;CWE-703;Other CVSS 'AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A double-free in the SMB client SMB2_flush replay path can occur because resp_buftype and rsp_iov keep stale response buffer state across replay attempts. A malicious or compromised SMB server can return a replayable flush response and then cause a later retry to fail before cifs_send_recv stores a replacement response, so flush_exit may free the old response pointer again. For the CVSS the PR:N is used because the attacker does not need a local account or privileges on the client when acting as the SMB server or a network attacker able to tamper with the SMB session. Base impact is at least denial of service via kernel crash, while the paranoid score treats the kernel double-free as memory corruption with possible confidentiality, integrity, and availability impact. The issue is network reachable in deployments where the vulnerable client mounts or accesses an attacker controlled or compromised SMB share, though practical exploitation likely requires precise replay and failure conditions.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8) YES DANGER DISK UAF SERVERTOCLIENT KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS YES NO checked ====================================================================== ACTIONABLESCORE ANALYSIS ====================================================================== ActionableScore=8 ActionableScoreLower=6 ## 1. ActionableScore * Conservative score: 6 * Paranoid score: 8 * Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum** ## 2. Signal breakdown Conservative signals: * Remote reachable / network-triggerable: +2. A malicious or compromised SMB server can influence SMB2 flush replay responses over the network. * Generic memory corruption, strong corruption primitive: +2. The commit explicitly fixes a double-free. * Real lifetime corruption: +1. Stale response buffer bookkeeping survives across replay attempts and can be freed again. * Reliable kernel crash / strong DoS: +1. Double-free in kernel SMB client response handling is a credible crash path. * Important filesystem/storage path: +1. This is in the SMB client filesystem path. * Special replay and retry failure conditions: -1. Triggering requires a replayable flush response followed by a retry failure before replacement response state is stored. Conservative total: 6 Paranoid additions: * Weak LPE concern: +1. Kernel double-free can sometimes become exploitable beyond DoS, although no reclaim or controlled replacement primitive is shown here. * Confidentiality impact plausible: +1. Only for paranoid handling due to memory corruption class. * Integrity impact plausible: +1. Only for paranoid handling due to memory corruption class. * Practical cap for high-complexity network client double-free: final paranoid score kept at 8 rather than 9 because exploitation depends on replay/error sequencing and no controlled reclaim primitive is demonstrated. Paranoid total: 8 ## 3. Reachability analysis The likely attacker is a malicious SMB server, compromised SMB server, or network attacker able to tamper with an SMB session. The victim side is the Linux SMB client. No local account on the victim is needed if the client already mounts or accesses the attacker-controlled share, so attacker-side PR is effectively none. The path is not an Internet-listening server path on the victim. It is network reachable in the client-to-server trust direction. Realistic exploitation requires the victim to connect to a malicious or compromised SMB endpoint and requires specific replay behavior plus a retry failure before `cifs_send_recv()` replaces the response pointer. Namespaces and containers do not materially reduce the remote-server attack model, but creating the SMB mount itself is usually privileged unless an existing mount, automount, desktop file manager, or managed environment causes access to the share. Call-site confidence: high. The patch and commit directly identify `SMB2_flush()`, the replay loop, stale `resp_buftype` and `rsp_iov`, and the double-free cleanup path. ## 4. Severity interpretation This is at least an Actionable Moderate issue because it is a network-influenced kernel double-free in a filesystem client path. It should not be auto-closed as an ordinary Moderate. Realistically, the clearest impact is kernel crash or DoS. Theoretical exploitation beyond DoS is plausible because double-free is a memory corruption class, but the patch does not demonstrate attacker-controlled reclaim, object replacement, function pointer control, arbitrary write, or a reliable LPE chain. That keeps the conservative score below high-end Important, while the paranoid score is high enough for manual review. ## 5. One-sentence report phrase A double-free in the Linux SMB client SMB2_flush replay path can be triggered by a malicious or compromised SMB server through stale response buffer state across replay attempts, causing at least kernel DoS and requiring manual review for possible memory-corruption impact. ## 6. Manual review recommendation MANUAL CHECK REQUIRED Reason: this is a network-influenced kernel double-free in the SMB client. Even though practical exploitation likely requires specific replay and failure conditions, the memory-corruption class and remote server influence make it unsafe to auto-close. ====================================================================== UPSTREAM PATCH SUMMARY ====================================================================== Patch: smb: client: fix double-free in SMB2_flush() replay Commit: 6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb Commit description: SMB2_flush() keeps its response buffer bookkeeping across replay attempts. If a replayable flush response is received and the retry then fails before cifs_send_recv() stores a replacement response, flush_exit will free the stale response pointer a second time. Reinitialize resp_buftype and rsp_iov at the top of the replay loop so cleanup only acts on response state produced by the current attempt. This fixes a double-free without changing replay handling for successful requests. Fixes: 4f1fffa ("cifs: commands that are retried should have replay flag set") Cc: stable@vger.kernel.org Reported-by: Yuan Tan Reported-by: Zhengchuan Liang Reported-by: Xin Liu Assisted-by: Codex:GPT-5.4 Acked-by: Henrique Carvalho Signed-off-by: Zhao Zhang Signed-off-by: Ren Wei Signed-off-by: Steve French Signed-off-by: Greg Kroah-Hartman Changed files: fs/smb/client/smb2pdu.c Diff excerpt: Not included in this email. See the upstream URL for the full patch. Full patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e27f40b682a5e42a2daae3ce6d96f0e0e16dedb ====================================================================== DETAILED REPORT METHODOLOGY ====================================================================== The original Linux kernel CVE announcement for CVE-2026-64383 can be found here: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64383 The original CVE announcement normally does not include a security-level estimate. In particular, it may not contain a CVSS assessment, an impact level, or enough information to determine whether the reported bug is a practically relevant security issue. One purpose of this parallel CVE list is to provide that missing technical and prioritization information. The original goal of the AL-KERNEL project was to prioritize Linux kernel CVE analysis automatically before manual review. The system can also help identify non-security issues that may be suitable for automatic closure. This report was generated by AL-KERNEL, an AI-assisted Linux kernel vulnerability analysis system developed by Alexander Larkin. The first analysis stage combines an autonomous classifier with additional LLM-based analysis. The autonomous classifier runs locally on a CPU and is based on a backpropagation neural network. Together, these mechanisms produce a technical vulnerability description, identify likely weakness types, estimate CVSS severity, and provide input for ActionableScore. Two CVSS estimates are retained because incomplete kernel vulnerability information often permits more than one defensible interpretation: Conservative CVSS vector: AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H The Best / paranoid CVSS score: 8.1 The conservative vector represents a lower-impact interpretation. The Best/paranoid vector intentionally represents a plausible upper-bound interpretation and should not automatically be treated as demonstrated real-world impact. CVSS may also need to be adjusted for a particular Linux deployment, because actual reachability, privileges, enabled kernel configuration, hardware, namespaces, exposed device nodes, and other environmental conditions can differ significantly between systems. A separate ActionableScore mechanism evaluates practical remediation urgency. Its analysis may include reachability, attack prerequisites, subsystem exposure, memory-corruption characteristics, denial-of-service reliability, and possible confidentiality, integrity, or privilege-escalation impact. Conservative ActionableScore: 6 Paranoid ActionableScore: 8 The final base severity is taken directly from the second tab-separated field of the AL-KERNEL classification result. ActionableScore does not replace or independently override that final AL-KERNEL decision, and if ActionableScore adjusted impact level of ALKERNEL, then you would see self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7. For an AL-KERNEL result of MODERATE, this report uses the following additional presentation split: ActionableScore below 5 -> MODERATE REGULAR ActionableScore 5 or more -> MODERATE 7.0 The distinction between MODERATE REGULAR and MODERATE 7.0 makes it possible to identify Moderate issues that should receive manual analysis and fixes before lower-priority MODERATE REGULAR issues. In many cases, MODERATE REGULAR fixes may wait for a later rebase or routine update. There is one override in which MODERATE REGULAR becomes MODERATE 7.0 even when the ActionableScore is below 5. When the AL-KERNEL result contains the KPANIC flag, a MODERATE result is always presented as MODERATE 7.0. The KPANIC flag selected with few regexps without usage of AI at all, so it helps to detect cases when Kernel Crash happens and similar (to filter False-Negative results from the LLM usage). KPANIC indicates that a reliable kernel crash, kernel panic, or similarly serious kernel availability impact was identified by the classification workflow. AL-KERNEL base severity for this report: IMPORTANT KPANIC detected for this report: YES Published priority for this report (same as in Subject): IMPORTANT These results are intended to support engineering triage. They are machine-generated estimates, and cases marked for manual review should be validated by a human security engineer before final disposition. For more info read docs linked from here: https://kernelcve.org/ (and you can submit you own patch there to generate such a report for non-existant CVE-id yet). Note that in many cases this AI tool selects higher severity, than real is (means you can expect Importants instead of Moderate 7.0 or Moderates 7.0 instead of regular Moderates). If you see such cases, please use reply email interface to add additional manual analyses info to this particular CVE. And please, please, let me know when you see Lows instead of Importants or Important instead of Low (because particular for such cases I need to tune this AI tool to make it better for this one and next similar). My contact email for such notifications is alexanjelausa@gmail.com (and both send reply to CVE record itself too and see "reply" button below for howto reply).